Dictionary of Evidence-based Medicine.pdf

122 Dictionary of Evidence-based Medicine
where A, is the mean rate, t is the time and o(St) is a value which tends to
zero as the small time interval tends to zero; (ii) the probability of more
than one event occurring during a small interval is equal to o(St); (iii) the
occurrence of events after any time t is independent of the occurrence of
events before time t. The Poisson process described above is referred to as
being stationary because of the assumed constant mean rate. The process
can be extended to model more complicated processes such as by using
mean rates which vary with time (non-homogeneous Poisson process).
Population at risk
The population at risk denotes the denominator in the calculation of event
(e.g. death, disease or adverse reaction) rates in epidemiological studies.
For example, when calculating the death rate for a particular community,
the population at risk is usually taken as the estimate of the population of
that community thought to be alive on July 1 of the calendar year concerned.
In estimating the incidence rate of an adverse drug reaction, the
population at risk is the number of patients exposed to the drug and
observed for the adverse reaction.
PORT (see Patient outcomes research team)
Positive economics
Positive economics is an approach to economic analysis which is only
concerned with the consequences of different changes or policies and does
not make judgements about the desirability of alternative allocation of
resources. This is in contrast to normative economics which is concerned
with analysing the desirability of different changes and policies. Normative
economics is also referred to as 'welfare economies'. The normative
approach is used when criteria are required to rank different options
(Johansson PO (1991) An introduction to modern welfare economics. Cambridge
University Press, Cambridge).
Positive rate of time preference (see under Discounting)
Post-randomization consent design
In a conventional randomized controlled trial (RCT), informed consent
is sought from patients to participate in the trial. Those who agree to